In order to purify a NOx (nitrogen oxide) in an exhaust gas, a diesel engine detects a NOx concentration in the exhaust gas by using a NOx sensor disposed at an exhaust passage, and adjusts the injection amount of urea water and diagnoses a catalyst and the like by itself based on the detection value.
In principle, the NOx sensor has a reference air chamber where a certain reference oxygen concentration is maintained, and a reference electrode disposed in the reference air chamber, in the sensor element configured by an oxygen ion-conductive solid electrolyte layer such as zirconia. Further, a reference pump cell is configured to maintain an oxygen concentration inside the reference air chamber to the reference oxygen concentration from the reference electrode and a pump electrode disposed outside the sensor element.
In the NOx sensor, in a case where the oxygen concentration of the reference air chamber cannot be maintained at the reference oxygen concentration due to the disturbance and the like, by the operation becomes failure and the NOx concentration which the NOx sensor measures becomes improper. Incidentally, the disturbance herein can be exemplified by the change of the oxygen concentration of the reference air chamber, the entry of moisture or hydrocarbon into the reference air chamber, and the like.
However, when a reference pump current flowing in the reference electrode increases in order to improve a tolerance against the disturbance, the deterioration of the reference electrode is promoted, and the control failure of the NOx sensor occurs. In addition, the oxygen of the reference air chamber goes around the heater to promote the deterioration of the heater, and thus the sensor element is not sufficiently heated.
With regard thereto, for example, as described in JP-A 2009-288082 (Patent Literature 1), there is proposed a method of controlling increase and decrease of the pump current of the sensor. In the control, in a case where the pump current is changed based on the constituent of the exhaust gas calculated from the operation state of the engine and then constituents such as hydrocarbon are increased, the pump current is increased to secure the reliability of the NOx sensor.
However, the above-described control does not correspond to a case where the moisture is condensed in the reference air chamber after a long-term storage of the vehicle, a case where the heater of the NOx sensor is heated, or the like. Thus, the NOx sensor is not maintained in a proper state in the long term.
Even if the oxygen concentration of the reference air chamber is maintained at the reference oxygen concentration, the constituent of the exhaust gas may cause the increase of the reference pump current, thereby progressing the deterioration of the reference electrode or the deterioration of the heater.